Generally speaking, every solicitation Mattel makes seems to work for me, though there are a few figures that I think need to be made fast!

-Ra's al Ghul

Classic, with shirt, cape and scimitar.

-The Joker

Really deserves an excellent, fully articulated sculpt with a sensible paint job. I think the trench coat/fedora look would be great at least for a varient, - this hardly ever gets made. Accessories should also be worthwhile- cane, revolver and pack of cards?

-Lex Luthor

Either in jumpsuit, lab coat or a three-piece suit; anything but armour.

-The Question

Vic Sage and Montoya versions. Just a great character with a truly iconic look.